ipn/ipnlocal: validate ping type (#8458)

Correct a minor cut-n-paste error that resulted in an invalid or
missing ping type being accepted as a disco ping.

Fixes #8457

Signed-off-by: Val <valerie@tailscale.com>

ipn/localapi/localapi.go

@@ -1331,7 +1331,7 @@ func (h *Handler) servePing(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	pingTypeStr := r.FormValue("type")
-	if ipStr == "" {
+	if pingTypeStr == "" {
 		http.Error(w, "missing 'type' parameter", 400)
 		return
 	}